This invention relates to electromagnetic wave absorbing silicone rubber compositions having a good electromagnetic wave absorbing ability and flexibility.
With the advance toward a higher density and higher integration of CPU, MPU, LSI and other components used in electronic equipment such as personal computers and mobile phones, the generation of electromagnetic noise now poses technical and social problems. The traditional countermeasure to electromagnetic disturbances is to use electromagnetic shields made of electroconductive materials to prevent electromagnetic waves from entering the equipment interior and from emanating from within the equipment interior. The electromagnetic shielding of this type can induce malfunctions because electromagnetic waves confined within the equipment interior give rise to electromagnetic interference.
In the prior art, an artisan with specialized knowledge and experience of noise suppression must be engaged in taking a countermeasure against disturbances by electromagnetic interference. It is a time-consuming task to find an effective countermeasure. Another drawback is that an electronic component in question must be previously given an extra space for mounting a shield.
To solve these problems, engineers are interested in electromagnetic absorbers which absorb electromagnetic waves for thereby reducing reflected and transmitted waves. Known electromagnetic absorbers include sintered soft ferrite and composite materials obtained by dispersing soft magnetic powder in matrices such as rubber and resins. The sintered soft ferrite is brittle and difficult to process, and the range of its application is limited because it suffers a sharp decline of its electromagnetic absorbing ability in a high frequency region. On the other hand, the composite materials having electromagnetic wave-absorbing soft magnetic powder dispersed in matrices such as rubber and resins are easy to process, but difficult to fill the soft magnetic powder to a high density, often failing to provide a high electromagnetic wave absorbing ability. Even if a high packing density is achievable, the resulting electromagnetic absorbers become hard and brittle and hence, very difficult to handle. Especially when the soft magnetic powder is of a metal base material such as iron or iron alloy, high packing is difficult because the powder is poorly wettable with silicone.
The same trend toward a higher density and higher integration of CPU, MPU, LSI and other components used in electronic equipment encounters the problem of increased heat release. Ineffective cooling will cause thermal runaway or undesired effects, giving rise to malfunction. One typical means for effectively radiating heat to the exterior is to dispose heat transfer media such as silicone grease and silicone rubber filled with heat conductive powder between CPU, MPU or LSI and heat sinks for reducing the contact thermal resistance therebetween. This means, however, cannot avoid the problem of electromagnetic interference within the equipment interior.
JP-A 2000-101284 discloses an electromagnetic absorber comprising an electromagnetic wave absorbing layer containing soft magnetic particles, a binder and an organic silane compound. It is described nowhere to use rubber as the binder. The composition described therein is effective for increasing the strength, but does not allow for high loading of soft magnetic particles.
When it is desired to have both an electromagnetic wave absorbing ability and a heat transfer ability, a soft magnetic powder and optionally, a heat conductive powder must be dispersed in a matrix such as rubber or resin. To impart a satisfactory electromagnetic wave absorbing ability and a satisfactory heat transfer ability, in particular, the increased loading of such powders is indispensable, but difficult with the state-of-the-art technology.
An object of the invention is to provide an electromagnetic wave absorbing silicone rubber composition having a satisfactory electromagnetic wave absorbing ability as well as improved workability and flexibility. Another object of the invention is to provide an electromagnetic wave absorbing silicone rubber composition having both a satisfactory electromagnetic wave absorbing ability and a satisfactory heat transfer ability as well as improved workability and flexibility.
It has been found that the above object is achieved by blending a soft magnetic powder, especially a soft magnetic powder of iron or iron alloy in silicone rubber and by further blending a specific surface treating agent therein for allowing the powder to be loaded in a larger amount. The same effect is achievable when a heat conductive powder is additionally blended in the silicone rubber. The surface treating agent used herein is selected from among (a) an organopolysiloxane containing at least one silicon atom-bonded alkoxy radical, silicon atom-bonded hydroxyl radical or functional organic radical in a molecule, (b) a titanate coupling agent, and (c) an aluminum coupling agent.
Specifically, when the surface treating agent selected from the above (a), (b) and (c) is blended in an electromagnetic absorber having a soft magnetic powder, especially a soft magnetic powder of iron or iron alloy, dispersed in silicone rubber, an electromagnetic wave absorbing silicone rubber composition is obtained which possesses a satisfactory electromagnetic wave absorbing ability and is easily workable and flexible.
Also, when both a soft magnetic powder, especially a soft magnetic powder of iron or iron alloy, and a heat conductive powder are dispersed in silicone rubber, and the surface treating agent selected from the above (a), (b) and (c) is blended therein, an electromagnetic wave absorbing silicone rubber composition is obtained which possesses both a satisfactory electromagnetic wave absorbing ability and a satisfactory heat transfer ability and is easily workable and flexible.
Briefly stated, the electromagnetic wave absorbing silicone rubber composition of the invention is arrived at by blending a soft magnetic powder in silicone rubber and further blending a surface treating agent. A preferred embodiment of the composition is arrived at by blending a soft magnetic powder and a heat conductive powder in silicone rubber and further blending a surface treating agent. In this embodiment, the composition in the cured state preferably has a thermal conductivity of at least 2.0 W/mK. The surface treating agent used herein is selected from among (a) an organopolysiloxane containing at least one silicon atom-bonded alkoxy radical, silicon atom-bonded hydroxyl radical or functional organic radical in a molecule, (b) a titanate coupling agent, and (c) an aluminum coupling agent.
The soft magnetic powder in the electromagnetic wave absorbing silicone rubber composition is preferably iron or an iron alloy. Soft magnetic materials are generally divided into ferrite base materials and metal base materials. The ferrite base materials exhibit a good electromagnetic wave absorbing ability only in a relatively low frequency region and so, their application is somewhat limited. Then the metal base materials are preferable. Among the metal base materials, iron and iron alloys are more preferable because they keep a good electromagnetic wave absorbing ability up to a relatively high frequency side and are inexpensive. Illustrative, non-limiting, examples of the iron alloy include Fexe2x80x94Cr, Fexe2x80x94Si, Fexe2x80x94Ni, Fexe2x80x94Al, Fexe2x80x94Co, Fexe2x80x94Alxe2x80x94Si, Fexe2x80x94Crxe2x80x94Si, and Fexe2x80x94Sixe2x80x94Ni alloys. The soft magnetic powder may be of one type or a mixture of two or more types. The soft magnetic powder particles may be either of flat or granular shape or a mixture thereof.
The soft magnetic powder (particles) should preferably have a mean particle size of about 0.1 xcexcm to about 100 xcexcm and especially about 1 xcexcm to about 50 xcexcm. Particles with a particle size of less than 0.1 xcexcm have too large a specific surface area, probably failing to achieve a high packing density. With a particle size of more than 100 xcexcm, the electromagnetic wave absorbing ability of soft magnetic powder per unit weight may become insufficient, especially on the high frequency side, fine asperities develop on the surface of the silicone rubber composition, and the contact thermal resistance become high when the heat transfer ability is necessary.
In a preferred embodiment, the soft magnetic powder is blended in an amount to account for 5 to 80%, especially 20 to 70% by volume of the entire silicone rubber composition. Less than 5% by volume of the soft magnetic powder may fail to impart the desired electromagnetic wave absorbing ability whereas more than 80% by volume may result in a silicone rubber composition which is hard and brittle. Preferably, the silicone rubber composition is cured into a part capable of achieving a noise attenuation of more than about 5 dB, especially more than about 10 dB at the desired frequency, when used with electronic equipment components.
When the silicone rubber composition is used in an area where heat transfer is necessary, a heat conductive powder is preferably used in combination with the soft magnetic powder in order to provide a high heat transfer capability. In this embodiment, the silicone rubber composition in the cured state preferably has a thermal conductivity of at least 2.0 W/mK and especially at least 3.0 W/mK.
The heat conductive powder used herein is typically selected from metals such as copper and aluminum, metal oxides such as alumina, silica, magnesia, red iron oxide, beryllia, and titania, metal nitrides such as aluminum nitride, silicon nitride and boron nitride, and silicon carbide, though not limited thereto.
Preferably the heat conductive powder has a mean particle size of about 0.1 xcexcm to about 100 xcexcm, especially about 1 xcexcm to about 50 xcexcm. Particles with a particle size of less than 0.1 xcexcm have too large a specific surface area, probably failing to achieve a high packing density. With a particle size of more than 100 xcexcm, fine asperities may develop on the surface of the silicone rubber composition, and the contact thermal resistance become large.
The heat conductive powder is used for the purposes of achieving closer packing with the soft magnetic powder and increasing the thermal conductivity of the composition. The amount of the heat conductive powder is preferably 10 to 85% by volume of the entire composition. The amount of the soft magnetic powder and the heat conductive powder combined is preferably 15 to 90%, especially 30 to 80% by volume of the entire composition. If the amount of the soft magnetic powder and the heat conductive powder combined is less than 15 vol %, little improvement in thermal conductivity is made. If the amount of the soft magnetic powder and the heat conductive powder combined exceeds 90 vol %, the composition may become hard and very brittle.
In either embodiment, the electromagnetic wave absorbing silicone rubber composition contains a surface treating agent selected from among
(a) an organopolysiloxane containing at least one silicon atom-bonded alkoxy radical, silicon atom-bonded hydroxyl radical or functional organic radical in a molecule,
(b) a titanate coupling agent, and
(c) an aluminum coupling agent.
The organopolysiloxane containing at least one silicon atom-bonded alkoxy radical, silicon atom-bonded hydroxyl radical or functional organic radical in a molecule (a) is exemplified by those of the general formula (1) below, though not limited thereto. 
Herein, R1 is OH or R3. R2 is OH, an alkoxy radical of 1 to 6 carbon atoms (e.g., methoxy and ethoxy), or xe2x80x94(CH2)pNH2 wherein p is an integer of 1 to 10. R3 is a monovalent hydrocarbon radical of 1 to 16 carbon atoms, for example, alkyl radicals such as methyl and ethyl, alkenyl radicals such as vinyl and allyl, and aryl radicals such as phenyl, with methyl being especially preferred. Letter m is an integer of 1 to 100, preferably 5 to 80, more preferably 10 to 50, and n is 1, 2 or 3. Preferred examples of xe2x80x94SiR2nR33-n are xe2x80x94Si(OCH3)3, xe2x80x94Si(OC2H5)3, xe2x80x94Si(CH3)2OH and xe2x80x94Si(CH3)2C3H6NH2.
The titanate coupling agent (b) includes, for example, isopropyltristearoyl titanate and tetra(2,2-diallyloxymethyl-1-butyl)bis(ditridecyl)phosphate titanate, though is not limited thereto.
The aluminum coupling agent (c) includes, for example, acetoalkoxyaluminum diisopropylates, though is not limited thereto.
The surface treating agent is preferably used in an amount of 0.1 to 10 parts by weight, and especially 0.2 to 8 parts by weight per 100 parts by weight of the soft magnetic powder. With less than 0.1 part of the surface treating agent, the surface treatment of powder particles therewith may become insufficient to fill a large amount of the soft magnetic powder, and the silicone rubber composition may become hard and less flexible. With more than 10 parts of the surface treating agent, the content of the soft magnetic powder and optional heat conductive powder is accordingly reduced, sometimes failing to achieve the desired electromagnetic wave absorbing ability and heat transfer ability.
The method of treating the soft magnetic powder and heat conductive powder with the surface treating agent is divided into a direct method of treating the powder directly with the agent and an integral blend method of adding the agent during the mixing of silicone with the powder. The direct method includes a dry method of directly treating the powder using a mixer capable of applying high shear stresses such as a Henschel mixer or super-mixer, and a wet method of adding the powder to a solution of the agent to form a slurry which is admixed. The surface treating agent may be introduced into the electromagnetic wave absorbing silicone rubber composition according to the invention by any of the above-mentioned methods although the method is not limited thereto.
The silicone rubber compositions used herein include unvulcanized putty-like silicone compositions and in the case of cured type, rubber-like compositions and gel-like compositions, though are not limited thereto.
Where the heat transfer ability is necessary, an electromagnetic wave absorbing silicone rubber composition having a lower rubber hardness in the cured state is preferred for improving the close contact with electronic equipment components or heat sinks and reducing the contact thermal resistance at the interface. It is thus recommended to use, among others, silicone rubber compositions of the low hardness type and silicone gel compositions. The rubber hardness in the cured state is preferably up to 80, especially up to 50 in Asker C hardness.
In the unvulcanized putty-like silicone compositions, silicone rubber compositions or silicone gel compositions, the base polymer may be a conventional organopolysiloxane, preferably of the following average compositional formula (2).
xe2x80x83R4aSiO(4xe2x88x92a)/2xe2x80x83xe2x80x83(2)
Herein, R4 is independently a substituted or unsubstituted monovalent hydrocarbon radical, and xe2x80x9caxe2x80x9d is a positive number from 1.98 to 2.02.
In formula (2), R4, which may be the same or different, stands for substituted or unsubstituted monovalent hydrocarbon radicals, preferably having 1 to 10 carbon atoms, more preferably 1 to 8 carbon atoms, for example, unsubstituted monovalent hydrocarbon radicals including alkyl radicals such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, hexyl and octyl; cycloalkyl radicals such as cyclohexyl; alkenyl radicals such as vinyl and allyl; aryl radicals such as phenyl and tolyl; aralkyl radicals such as benzyl, phenylethyl and phenylpropyl; and substituted monovalent hydrocarbon radicals including the foregoing radicals in which some or all of the hydrogen atoms attached to carbon atoms are substituted with halogen atoms, cyano and other radicals, for example, halogenated alkyl radicals and cyano-substituted alkyl radicals such as chloromethyl, bromoethyl, trifluoropropyl and cyanoethyl. Of these, methyl, phenyl, vinyl and trifluoropropyl radicals are preferable. More preferably methyl accounts for at least 50 mol %, especially at least 80 mol % of the R4 radicals. The letter xe2x80x9caxe2x80x9d is a positive number from 1.98 to 2.02. Preferably the organo-polysiloxane has at least two alkenyl radicals per molecule, especially with the alkenyl radicals accounting for 0.001 to 5 mol % of the R4 radicals.
The organopolysiloxane of formula (2) may have any molecular structure and is preferably blocked at ends of its molecular chain with triorganosilyl radicals or the like, especially diorganovinylsilyl radicals such as dimethylvinylsilyl. In most cases, the organopolysiloxane is preferably a linear one although a mixture of two or more different molecular structures is acceptable.
The organopolysiloxane preferably has an average degree of polymerization of 100 to 100,000, especially 100 to 2,000, and a viscosity of 100 to 100,000,000 centistokes at 25xc2x0 C., especially 100 to 100,000 centistokes at 25xc2x0 C.
When the above silicone rubber composition is formulated to the addition reaction type, the organopoly-siloxane is one having at least two alkenyl radicals such as vinyl radicals per molecule, and the curing agent is a combination of an organohydrogenpolysiloxane and an addition reaction catalyst.
The organohydrogenpolysiloxane is preferably of the following average compositional formula (3):
R5bHcSiO(4xe2x88x92bxe2x88x92c)/2xe2x80x83xe2x80x83(3)
wherein R5 is a substituted or unsubstituted monovalent hydrocarbon radical of 1 to 10 carbon atoms, the subscript xe2x80x9cbxe2x80x9d is a number from 0 to 3, especially from 0.7 to 2.1, and c is a number from more than 0 to 3, especially from 0.001 to 1, satisfying 0 less than b+c xe2x89xa63, especially 0.8xe2x89xa6b+c xe2x89xa63.0. This organohydrogenpolysiloxane is liquid at room temperature.
In formula (3), R5 stands for substituted or unsubstituted monovalent hydrocarbon radicals of 1 to 10 carbon atoms, especially 1 to 8 carbon atoms, examples of which are the same as exemplified above for R4, preferably those free of aliphatic unsaturation, and include alkyl, aryl, aralkyl and substituted alkyl radicals, such as methyl, ethyl, propyl, phenyl, and 3,3,3-trifluoropropyl among others. The molecular structure may be straight, branched, cyclic or three-dimensional network. The SiH radicals may be positioned at an end or intermediate of the molecular chain or both. The molecular weight is not critical although the viscosity is preferably in the range of 1 to 1,000 centistokes at 25xc2x0 C., especially 3 to 500 centistokes at 25xc2x0 C.
Illustrative, non-limiting, examples of the organohydrogenpolysiloxane include 1,1,3,3-tetramethyldisiloxane, methylhydrogen cyclic polysiloxane, methylhydrogen-siloxane/dimethylsiloxane cyclic copolymers, both end trimethylsiloxy-blocked methylhydrogenpolysiloxane, both end trimethylsiloxy-blocked dimethylsiloxane/methylhydrogen-siloxane copolymers, both end dimethylhydrogensiloxy-blocked dimethylpolysiloxane, both end dimethylhydrogensiloxy-blocked dimethylsiloxane/methylhydrogensiloxane copolymers, both end trimethylsiloxy-blocked methylhydrogen-siloxane/diphenylsiloxane copolymers, both end trimethylsiloxy-blocked methylhydrogensiloxane/diphenyl-siloxane/dimethylsiloxane copolymers, copolymers comprising (CH3)2HSiO1/2 units and SiO4/2 units, copolymers comprising (CH3)2HSiO1/2 units, (CH3)3SiO1/2 units and SiO4/2 units, and copolymers comprising (CH3)2HSiO1/2 units, SiO4/2 units and (C6H5)3SiO1/2 units.
The organohydrogenpolysiloxane is preferably blended in the base polymer in such amounts that the ratio of the number of silicon atom-bonded hydrogen atoms (i.e., SiH radicals) on the organohydrogenpolysiloxane to the number of silicon atom-bonded alkenyl radicals on the base polymer may range from 0.1:1 to 3:1, more preferably from 0.2:1 to 2:1.
The addition reaction catalyst used herein is typically a platinum group metal catalyst. Use may be made of platinum group metals in elemental form, and compounds and complexes containing platinum group metals as the catalytic metal. Illustrative examples include platinum catalysts such as platinum black, platinic chloride, chloroplatinic acid, reaction products of chloroplatinic acid with monohydric alcohols, complexes of chloroplatinic acid with olefins, and platinum bisacetoacetate; palladium catalysts such as tetrakis(triphenylphosphine)palladium and dichlorobis(triphenylphosphine)palladium; and rhodium catalysts such as chlorotris(triphenylphosphine)rhodium and tetrakis(triphenylphosphine)rhodium. The addition reaction catalyst may be used in a catalytic amount, which is often about 0.1 to 1,000 ppm, more preferably about 1 to 200 ppm of platinum group metal, based on the weight of the alkenyl radical-containing organopolysiloxane. Less than 0.1 ppm of the catalyst may be insufficient for the composition to cure whereas more than 1,000 ppm of the catalyst is often uneconomical.
In the practice of the invention, silicone rubber compositions of the addition reaction curing type as mentioned above are preferred because they tend to cure to a lower hardness.
In the other embodiment wherein the silicone rubber composition is of the peroxide curing type, organic peroxides are used as the curing agent. The organic peroxide curing is useful when the organopolysiloxane as the base polymer is a gum having a degree of polymerization of at least 3,000. The organic peroxides used may be conventional well-known ones, for example, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, p-methylbenzoyl peroxide, o-methylbenzoyl peroxide, 2,4-dicumyl peroxide, 2,5-dimethyl-bis(2,5-t-butylperoxy)hexane, di-t-butyl peroxide, t-butyl perbenzoate, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclo-hexane, and 1,6-bis(t-butylperoxycarboxy)hexane.
An appropriate amount of the organic peroxide blended is about 0.01 to 10 parts by weight per 100 parts by weight of the organopolysiloxane as the base polymer.
In addition to the above-described components, the silicone rubber composition may further contain conventional additives.
Any conventional methods are employable for preparing and curing the electromagnetic wave absorbing silicone rubber composition according to the invention.
On use, the electromagnetic wave absorbing silicone rubber composition is molded and cured into a sheet. The sheet is typically disposed within an electronic equipment for suppressing electromagnetic noise within the equipment. Also, the electromagnetic wave absorbing silicone rubber composition to which is further imparted a heat transfer ability is molded and cured into a sheet, which is typically disposed between an electronic component and a heat sink in an electronic equipment for thereby suppressing electromagnetic noise and promoting heat transfer from the electronic component to the heat sink and hence, to the exterior.